The present invention relates to a transformer with a gap for switching power supply used in a switching regulator and an inductor with a gap, and more particularly to the transformer and the inductor which use divided bobbins.
Various transformers for switching power supply and inductors have been known. In those transformer and inductors, a measure has been taken for adjusting an inductance and preventing a magnetic saturation of the core. In the case of the EE type core, an air gap is formed at the abutting portion of the center leg of the core. When the transformer and inductor are operated, a magnetic flux leaks through the gap and couples with the coil located near the gap. The leaking magnetic flux causes an eddy current in the coil to heat the coil and possibly interferes with components located outside the transformer. Some measures to reduce the heat generated in the coil by the eddy current loss have been proposed. As shown in FIG. 7, a technique to locate the coil apart from the air gap 15 is proposed in Unexamined Japanese Patent Publication 2-44704. As shown in FIG. 8, a technique to eliminate the coil at a location near the air gap by providing a protruded portion at the location near the coil is proposed in Unexamined Japanese Patent Publication 7-302720. To shield the leaking magnetic flux, the outer peripheral surface of the core is usually covered with a shield ring of a copper plate as shown in FIG. 9. In some devices, a wire is used in place of the copper plate for the same purpose (Japanese Utility Model Nos. 2518250 (FIG. 10A) and 2518241 (FIG. 10B)).
Problems of the prior devices described above will be described.
The automization of the manufacturing of the transformer and the inductor is generally realized. In the manufacturing method in which the shield ring of a copper plate is provided on the side walls of the transformer or the inductor as shown in FIG. 9, the mounting and soldering by manual still occupy a major part of the manufacturing work. Such manual work requires a number of steps for its manufacturing, and hinders simplification and automization of the manufacturing process.
According to FIG. 7 (Unexamined Japanese Patent Publication 2-44704), the whole coil and the core are excessively large in size since the thick bobbin is used, and the coil is put on the thick bobbin.
According to FIG. 8 (Unexamined Japanese Patent Publication 7-302720), the protruded portion is provided at the location near the coil, and no coil is present at a location near the air gap. Therefore, the eddy current loss by the leaking magnetic flux is extremely reduced, but the magnetic flux leaked through the air gap propagates into the air to interfere with other components. To avoid the interference by the leaking magnetic flux, it is necessary to entirely cover the core with an additional shielding means, for example, a copper plate as shown in FIG. 9.
In the case of FIG. 10A, the chamber is provided around the upper collar portion 70 of the bobbin, and the shield coil 74 of a wire is wound therearound. That is, the wire shield coil 74 is used in place of the copper plate shield ring. The winding beginning and ending ends of the shield coil are connected to the terminal 71 buried in the upper collar portion 70 of the bobbin in a shortcircuiting manner. To this connection, soldering is required for the terminal 71. Therefore, to complete the transformer, two steps of soldering are exercised, one for the connection of the terminal 71 and the other for the connection of the terminal 79 located on the side opposite to the side having the terminal 71 located. This leads to increase of the number of manufacturing steps. In the transformer of FIG. 10B, the shield coil 84 is formed like a short ring by use of a wire, the core and the bobbin are combined, and the shield coil is attached to on two shield coil receiving portions 81 provided on the upper collar portion 80 of the bobbin. Therefore, the fixing of the shield coil 84 is instable.
Further, the step of manufacturing the shield coil is additionally provided. The shielding method by use of the wire is effective in shielding the magnetic flux component of the transformer or the inductor which develops in parallel with the main magnetic flux, but is ineffective for the magnetic flux component perpendicular to the main magnetic flux and the magnetic flux leaking through the air gap.